Conformable Pressurized Structures: Design and Analysis

Abstract

There are many applications where volume needs to be pressurised within a geometrical space for which conventional pressure vessels do not provide suitable solutions. Applications are for example found in pressure cabins for Blended Wing Body Aircraft and conformable pressure vessels for an incompressible medium that has to be stored in a pressurised environment, specifically liquid gasses (e.g. propane) or cryogenic applications. These applications demand conformable pressurised structures but up until now, there are no conformable pressurised structures with full spatial freedom, made from high specific strength materials and optimal structural efficiency realised yet. The aim of this thesis is to provide understanding in the analysis and structural design of conformable pressurized structures. This thesis will enable the development of conformable pressure cabins and pressure vessels. From a structural topology point of view, efficient structures are in-plane, rather than out-of-plane, loaded structures. In this thesis, it is shown by means of linear membrane analysis that any arbitrary combination of intersecting axi-symmetrical membrane structures will always carry the pressurization loads via in-plane stresses provided that there is a proper in-plane loaded reinforcement at the surface of intersection. The most well-known example of a structure like this is a cluster of soap bubbles. In this example, the axi-symmetrical membrane structure is a sphere and the intersecting walls are the in-plane loaded reinforcements. This type of conformable pressure vessel is therefore called the multi-bubble. Simple shapes like intersecting spheres, intersecting cylinders or intersecting toroids are easy to analyse but the analysis becomes complex when different types of axi-symmetric elements are intersecting each other. This problem has been tackled by the visual interpretation of the linear membrane theory. To solve for loads and forces in the multi-bubble, it is shown that the solution simply depends on pressure and geometric variables. The solution is only valid however when the shell has the optimal stiffness distribution. The optimal stiffness distribution causes the inflated shape to be identical to the original shape, only the size is different. The optimal stiffness distribution is derived in this thesis. Manufacturing restrictions and cost limitations might cause the optimal stiffness distribution not to be economically feasible. A non-optimal stiffness distribution causes a nonlinear membrane response. A modified linear membrane solution for orthotropic membranes is derived in this thesis to solve for forces and deformations in non-optimal membranes. Furthermore, the understanding of the membrane response of pressurised structures also enables the formulation of a tailored stiffness distribution in order to control/eliminate deformations in particular directions. This can ease the integration of the pressurized structure in the surrounding structure. The achievements of this research can be summarized into: . Provision of a deeper understanding of the shape of (conformable) pressurized structures . Development of the method to analyse the forces at the surface of intersection of the intersecting membrane structures . Definition of the optimal stiffness distribution in order to derive the desired membrane response. . Formulation of a tailored stiffness distribution in order to control/eliminate deformations in particular directions. This can ease the integration of the pressurized structure in the surrounding structure. . Formulated the orthotropic version of the modified linear membrane theory in order to analyse the nonlinear membrane response in case the membrane’s stiffness distribution differs from the optimal distribution. . Presentation of conceptual ideas of conformable pressurized structures applied to pressure tanks and pressure cabins.